If I'd have to choose between buying Rhino4 and C4D10, I'll most likely have a hard time deciding which. Both are in the same price range. First of all, Rhino 4 is just a modeler. You still need a renderer like Flamingo.
Considering Rhino is mainly used for industrial/mechanical design offices, I won't have to buy it anyway if I were hired by a company who uses Rhino. Which means the free Rhino demo is enough for learning purposes.
Yup, if I have that kind of money, I'd finally pick C4D10.
But then, I'd have to wait till C6 is released.